This invention relates to a television synchronizing system which generates standard interlaced television synchronizing signals from external synchronizing signals which may be nonstandard.
Television cameras are required to generate synchronizing signals in synchronism with the video to be reproduced so that the video image may be maintained in proper position on a raster. Studio cameras ordinarily include elaborate sync signal generators. These sync signal generators are adapted for synchronization with a master source generated within the studio (gen locking).
In the field of industrial and surveillance cameras, cost considerations preclude complex sync generators. Nevertheless, it may be desirable to synchronize the internal sync generator to an external source. For example, in the simpliest systems it may be desirable to phase-lock the internal sync signal generator of a camera to a zero crossing of the sixty Hz power line. Thus, when several surveillance cameras are used in different locations and monitored from a central location, the monitored picture does not roll when switching from one camera to another. Unfortunately, other uses of industrial and surveillance cameras place more stringent requirements on the synchronizing capability of the internal generator of each camera than are experienced in studio camera uses. For example, industrial and surveillance cameras may be used in split-screen applications, necessitating both horizontal and vertical-frequency synchronization. Additionally, a camera may be required to be synchronized with a source of nonstandard television synchronizing signals for split-screen use, as with a home computer, a video game, or an old-style camera, any of which may generate noninterlaced signals or RS330 (lacking horizontal-rate serrations in the vertical synchronizing interval) signals, or noninterlaced RS330 signals.
Vertical count-down systems for generating interlaced vertical and horizontal synchronizing signals are described in U.S. Pat. No. 3,688,037, issued Aug. 29, 1972 to Ipri, and U.S. Pat. No. 3,878,335, issued Apr. 15, 1975 to Balaban and elsewhere. In such systems, the phase detector is coupled to the horizontal-rate signals developed by a synchronizing signal separator for controlling an oscillator and counter arrangement for generating horizontal-rate pulses in synchronism with the external horizontal-rate signals. Internal vertical-rate pulses are generated by a logic circuit coupled to a counter which is in turn coupled to the phase-lock loop. Attention in such prior art is directed to stabilizing the internally generated vertical synchronizing signal by the use of logic systems for identifying the external vertical synchronizing signal.
U.S. Pat. No. 3,866,584, issued Nov. 5, 1974 to Itoh and U.S. Pat. No. 4,144,544, issued Mar. 13, 1979 to Fernsler are directed to stabilizing the image displayed on the raster of a television apparatus when the source of signals is a VTR in which a step change in phase of the horizontal synchronizing signal occurs at a time just prior to the beginning of the vertical blanking interval. In such systems, in order to display an undistorted image on the raster, the internal horizontal synchronizing signal of the display device must be totally synchronized to the new phase of the horizontal synchronizing signal from the VTR by the time of beginning of scan (at the top of the raster), and this is accomplished by increasing the loop gain of the phase-lock loop by which the internal horizontal synchronizing signals are generated.
For split-screen use of a surveillance camera in conjunction with a VTR or other source of nonstandard signals, the internal sync signals of the camera may continue at a constant rate, since it is the display device rather than the camera which is arranged to slew to compensate for transient errors in the VTR sync signals. Consequently, distortions of the displayed image may occur if the horizontal synchronizing signals of the camera are locked to the horizontal synchronizing signals of the VTR, since it is the display apparatus which must slew to correct the image. Furthermore, when the source with which the camera is to be synchronized for split-screen use produces RS330 signals, the camera receives no horizontal synchronizing signals at all during the vertical synchronizing interval, and may drift away from the horizontal synchronizing signal phase. If, in addition, the external synchronizing signal source is noninterlaced, phase perturbations in the horizontal-rate signal may occur in the vicinity of the vertical synchronizing and blanking intervals. It is desirable that a camera produce standard synchronizing signals at the horizontal rate of an external source whether or not the external source produces a standard signal.